Volume compressor for carbon microphones



Dec. 27, 1949 J. c. OBRIEN VOLUME COMPRESSOR FOR CARBON MICROPHONES Filed July 16, 1946 Ilu- IQIII Patented Dec. 27, 19491 VOLUME COMPRESSOR Foa CARBON Y MIcRoPnoNEsH John C. OBrien, Rochester, Y

General Railway Signal Gompany,

N. Y., assignor to Rochester,

Application July 16, 1946, Serial No. 684,062

7 Claims. 1

This invention relates to audio volume compressor organizations, and more particularly pertains to the provision of a regulator organization for governing the output of an audio amplifier having associated therewith a carbon type microphone.

It is, of course, generally understood that the audio frequency output of a microphone must be amplied in order to suitably act upon the modulating stages of a radio transmitter or for the control of recorders, and the like. However, in both cases, it is desirable to limit the variation in the volume of the audio output of the amplifier even rthough the input to the microphone may Vary over a wide range. In the case of radio transmission, such a volume compression is desirable to prevent over-modulation which would cause distortion. Similarly, in making recordings, there are limits to the recordingchannel regardless of the type of recorder employed.

In view of the above considerations, it is proposed in accordance with the present invention to provide an audio volume compressor organization suitable for use with a carbon type microphone especially adaptable to portable equipment.

Generally speaking, and without making any attempt to dene the exact nature or scope of the invention, it may be stated that the compressor organization contemplated includes two elec-V tronic tubes one of which acts as an amplifier and the other of which acts as a regulator tube to govo ern the current through the microphone. This regulator tube by decreasing the current iiowv through the microphone upon an increase in the output of the amplifier tube acts to decrease the audio frequency output of the microphone at the input of the amplier. In this way, the microphone is limited in the extent of the volume that it. can supply to the amplifler even though there may be wide variations in the input supplied to the microphone.

"A further feature' of the present invention is to K organize the amplifier and regulator tubes in such a'way as to provide control over the bias of the grid 'of the amplier in accordance with the output of the amplier. This acts to vary the amplication factor of the amplier tube in a manner to further compress the volume variations with respect to the local carbon microphone. Also,

remote point over line wires (or other source of audio input).

Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings, and in part pointed out as the description of the invention'progresses. I

In describing the invention in detail, reference will be made to-t'he accompanying drawing which illustrates a volume compressing organization in a simplied diagrammatic manner as one embodiment of thepresent invention.

For the purpose of simplifying the illustration and facilitating in the explanation, the various parts andfcircuits constituting the embodiment of the invention have been shown diagrammatically and certain conventional illustrations have been employed, the drawing having been made Y more with the purpose of making it easy to understand theyprinciples and-mode of operation than with the idea of illustrating the specific constructions and arrangement of parts that would be employed in practice. Thus, the various devices are illustrated in a conventional manner, and various conventional symbols are used to indicate the devices, wiring connections, batteries or other sources of-electric current and the like.

With reference to the single figure of drawing, an audio volume compressor .organization is shown as including an amplifier tube TI and a regulator tube T2 having associated resistors, capacitors, wiring connections and the like so arranged as to supply an audio output at terminals 5 .and 6, which audio output may be connected to another amplier stage or may be connected di- Yconnection with a microphone of the carbon type.

this compression characteristic eiected by varial tion of the amplification factor of the amplifier can be used independently of the current control the local microphone to compress the volume variations of an audio input suppliedfr-or'n'a The ampliiier tube TI and regulator tube T2 have been shown as independent tubes having their cathodes connected together and providing a circuit to a suitable ground connection through .av-microphone, of the. carbon type. However,

"these two tubes may be combined in a single en- 3 velope and have a common cathode connection. but for convenience in the circuit diagram the tubes have been shown in separate envelopes. The cathode heaters for the tubes TI and T2 are assumed to be continuously energized from a suitable source. The plate supply has been indicated as a battery designated B having its negative terminal connected to ground and" itspositive terminal connected through a resistor Rl to the plate of the regulator tube T2. The plate supply B is also connected through resistors R2 and R3 to the plate of the amplier tubeTl. The

audio frequency output of the amplifier tuberTt Y is connected through a condenser Cl and resistor R4 to ground. The potential drop* across the' variable resistor R4 supplies'the output totermi'- nals 5 and 6.

A feed-back connection is provided to.l thegridiv of the regulator tube T2 by the condenser C2, and a grid leak resistor R6 is connected between such grid and ground. A resistor R5 provides a connection betweenthe grid of the regulator tube and the control grid of the amplifier tube TI, while a condenser C3 provides a connection to ground from the control grid of the amplifier tube Tl for audiofrequencies.

A transformer TF" is shown having its secondary winding connected between the grid of TI and the by-pass condenser C3 while its primary winding is. indicated by suitable legend as o connected to a remotely located microphone over a suitable line circuit.

A condenser C4 is provided to` by-pass audiofrequencies from the plate ofthe regulator tube T2 to the cathode for reasons later explained;

With the above general discussion of the structure of the organization, one# possible theory of' operation of the organization willbe given as ai guide to the practice of the invention.

With reference to the drawing, it will be apparent that the microphone is located in the plate-cathode circuit of' both the amplifier tube TI and the regulator tube T2. Since themicrophone is of the carbon type, it provides a variable resistance in the cathode circuits of the tubes- Tl and T2 to act as a variable bias for the grids of the respective tubes;

The variations in resistancev of the microphonealso acts to vary the voltage in thev plate circuit of the tube Tl, but since the audio alternating voltage output of such amicrophone is: relatively small, it will be appreciated that the effect of the microphone is mainly on the grid circuit of this tube Ti Thus, the tube TI- may be convenientlyr considered as a cathode driven or modulated ami plifier since the action of themicrophoneis upon both its grid and plate circuits.

It will be apparent thatv the same audofre-v quency voltages appearing across the condenser CI and resistor R4 also appear acrossv the resistances R2 and R3 in series with the condenser C4- Thus, a portion of the audio frequency output of the amplifier tube 'I-l is fed to the control grid of the tube T2 through a condenser C2 by a connection made between theresistorsrRZ- and R3. This audio frequency supplied to thel grid of the regulator tube- T2 is rectified by this grid and builds up a negative bias across the grid leak resistor R6. Since the grid leak resistor R6 tends to provide a drain for suchy negative bias, the actual value of' negative bias depends` upon the value of the audio signal output of the amplifier tube TI.

The negative bias built up across the grid leak resistor R6 is also supplied' to the control grid 'signal the greater is of the amplier tube Tl through the decoupling resistor R5 and with a by-pass condenser C3. In other words, the negative bias supplied to the control grid of the regulator tube T2 is also supplied to the control grid of tube TI with only a slight lag due to the capacity of condenser C3 and the resistance value of the resistance R5.

It will be. noted that the plate supply battery B' is fedY directly to the plate of the regulator tube T2 including only the resistor Rl, whereas the plate circuit of the ampliiier tube Tl also includes resistors. R2 and R3. For this reason, the plate-cathode current of the amplier tube Tl is relatively small compared to the plate-cathode current of the regulator tube T2. These currents reach their combined maximum value in owing through the microphone when there is no audio input 'to the microphone. Thus, any message spoken into the microphone, or sound otherwise produced, causes an audio signal to be applied to the grid of the amplifier tube TI which audio signal is amplified in accordance with the then existing biasl onl the control grid' oi that tube. vThis output of the amplifier Tl is fed to: thelter networkv associated with the control grids to producel a' negative bias of a value depending upon that strength signal within a timev factor determined' by the relative sizes of the condensers and resistors4 involved; It might be noted; that the time delay response in the change inV bias on these tubes is preferably in the order of a fractionY of a second', such as 1/zuof a second or the like, so that variations in volume supplied to the microphone will result in4 being'compressed' if such variations exist longer than the time delayY inherently built into the organization. but wll'not be erased if the variations are relatively shorter. This is a desirable' characteristic so as to retain a reasonable amount of the inections of the voice andthe emphasis of the words spoken to thereby give a suitable degree of naturalness to the reproduced signal.

It willv be apparent that any increase i'n the negative bias on the control grid of the regulator tube T2 results in a decrease of'its plate current which decreases the current iowing through the microphone. Such decrease in current in the microphone directly aiects its input with respect to the control grid of the amplifier tube Tl'. In. other words, the stronger the audio frequency through the microphone.. This acts to, compress. the variations in intensity oi" audio signal. by governing the current through the microphone..

The condenser C3` and resistor R5 actA as. a l'ter combination to prevent audio Yfrequency signal from being applied to the control grid of the amplii'ier tube TI, since such an application of audio frequency signal would' cause regenera@ tion andf distortion of the outputofthe. ampli'ier.

The ampli'er tube Ti can to advantage be off the variable-mu or remote cut-01T control' grid type so that an increase in the negative bias, on suchv control grid will cause a decrease in the amplication factor of the tube. In this way the increase in negative bias supplied by the circuit organization to the two control grids acts to cause compression both by a regulation. of' the current through the` microphone and by a decrease in the amplification factor of the amplifying tube Tl. However, in this regard it should. be noted' that the majorA portion of' the regulation is supplied by the control of the current through the microphone and that the variable-mu. characteristics' of the amp'lier tube act as supthe decrease in the current..

. organization.

plementary to the desired characteristics.- of theorganization. l

In this connection, it should be noted that the control gridof tube Tl can be connected directly to ground if the compression due to the control of the ampliiication factor of tube Tl is not desired, since the compression given by the regulator tube T2 is substantial and provides the major portion of the regulation of the output with respect to the phone. However," in the present organization, it is desired to also provide for an input to the an'ipliierVv tube Tl microphone. e Y, Y l

The output of this remote microphone is contransformer such as TFinto the grid circuit of the tube TI without in any Way affecting the operation ofthe local microphone. The remote microphone, thus connected, may have its variable input governed in accordance with the variable-mu characteristics of the tube Tl but not by the regulator tube T2. However, this is entirely feasible because it is contemplated that such remote microphones are usually located in an oince or studio where 'the lrange of volume variations will be less than that which may occur in connection with the portable microphone local to the compressor organization. This is especially true where the organization is employed in connectionwith railroad communication systems in which case the remote microphone will probably be in an oilce used by the same operator rather consistently, whereas the local microphone may be adjacent to the trackway or in a locomotive cab and used by many different trainmen under varying conditions.

The volume compressor organization provided in accordance with the present invention is thus very useful in train communication systems since it is an aid in reducing background noise and interference. For example, when the background noise is very high, the trainman will speak very loudly into the microphone in an efort to overcome the local background noise. In so doing, the compressor action takes place limiting the amplification of the organization with respect to the voice message spoken into the microphone. In so doing, it actually reduces the amplification to such an extent that the background noise is not reproduced to any substantial degree, whereas the voice message is reproduced with just a silghtly increased volume. In other words, the use of the compressor organization of the present invention acts to eliminate background noises to the extent that they are compensated for by the trainman in regulating his voice in accordance with such background noises.

The condenser C4 is provided to by-pass audio frequencies between plate and cathode of the tube T2 so that any audio frequencies which might be produced by the feed-back connection to the control grid of the tube T2 will be suppressed and not cause regeneration in the circuit For the same reason, the decoupling resistor Rl is provided to keep the audio frequency of the plate circuits from being fed back to the grid circuit of the tube TI via the plate supply battery B.

In an organization of this type, it is necessary that the plate supply potential be held substantially constant so as to be able to properly adjust the different values in the compressor organization to their operating points for giving the desired operation. The usual battery, such asY designated B, will supply a suitable constant polocal carbon type -microfrom a remotely .locatedV nected over a suitable line circuit and through a' tential, but it should be understood iii-'actual practice suitable power pack will probably be employed, and in such a case it is desirable that the power supply organization be of sucha character as to give a substantially constant plate potential. This can be accomplished by numerous.

expedients such as by lator gas tube. Y

kIt should be-understood that the coefficients of thetubes T l and vT2 may be suitably chosen in accordance with the requirements of the particular application in practice to which the comprressor` organization is to be applied. If mote'inputis not tobe employed, then it is apparent that the variable-mu characteristics of the tube TI are hand, if it is desired to provide considerable compression for the remote input as well as additional compression for the local microphone input, then the tubeTl should bev of a remote cut-off type. I

Having described an audio volume compressor organization as one specific embodiment of the present invention, it is desired to be understood that this form is selected to facilitate in the -theuse of a voltage regudisclosure of the invention rather thanv to limit` the number of forms which it mayr assume; and, itis to be further understood that various modifications, adaptations [and alterations may be applied to the speciiic form shown to meet the requirements of practice, rwithout Vin any manner departing from the spirit or scope of the present invention.

What I claim is:

l. In an audio volume compressor organization, a carbon microphone, an ampliiier connected to be cathode modulated by said microphone, a regulator tube having its cathode-plate circuit also connected through said microphone, and means governing the output of said regulator tube to decrease the current through said microphone when the output of said amplier increases to thereby compress the audio variations in volume as supplied to said microphone.

2. In an audio volume compressing organization, a carbon microphone, an amplifier stage and a regulator stage both having their cathodeplate circuits connected through said microphone, whereby said microphone can provide an audio component for the grid of said amplier, means for causing the plate current of said amplier to be relatively small compared to the plate current of said regulator, and feed back circuit means connected from the plate of said amplifier to the grid of said regulator for increasing the negative bias on said regulator as the output of said amplii'ler increases to thereby decrease the audio output of said microphone into the grid cathode circuit of said amplifier and thereby compress the variations in volume supplied to said microphone.

3. In an audio volume compressing organization, an amplifier having an audio input supplied by a carbon microphone, the output of said microphone varying in accordance with its current supply, means including a regulator stage for controlling the current through said microphone, and means for governing the output of said regulator stage in accordance with the output of said amplier to cause a decrease in microphone current as the output of said amplifier increases.

4. In an audio volume compressor organization, a carbon type microphone, an amplifier connected so as to be cathode driven by said microphone and having a variable-mu control grid,

a regulator tube having its cathode-plate circuit the renot necessary. On the other,4

connected: throught-saidemierophone and having az-ccontrol-Ygrld togovemr thefoathodmplatefcireuit currenta.feed-back-n circuit'- for supplyinga asl por tionzofffthewutputf oi4` saidflamplienf-to 4the-oon-A trolfgrid'foffsaid regulatortubei al gridvleak filtern ci'rcuil'f'connectedi'toutlrecontroll gridof said regulater! tube; and# also connecting# to-saidi control7 grid of said amplifier with decouplingl'withrespect.' tin-.audiofrequencieswhereby-'the-increaseofaudio input: to-V said micropfl'ione2` causes-` ar increased.s negative bias toibo-placed? on Y the control 1 grids offsaidif tubes. to thereby decrease-the amplifica tiony factor off the entire organization if bothby; decreasing tthe current through saidf' microphone and 2by-decreasing-the -ampli'cationfl-factor ofL 'saidf amplifier:

5;" Ina-naudiovolume-compressor-organization; al carbon microphonel and` anfassociated; audio amplifier; vsaid-L carbon microphone`v supplying; arr; audioinputf tof saidamplifierA in accordance with theivalue' offcurrerrtnow-through it; and regulator means* governed by the output of'- said` amplifier for, governing the current; ow through said,v microphone to in turn control the audioinput'to said"ampl1er from saidl microphone 6;. In a, volume compressing. Organization. .am audio ampliier includL gta .tube oictn'e variable-v mu, .type ,hayinga nl jateA Qutputcircuitandanon: tmlgrd. ,means for..summing, a,modulating:T fre:v

quencxtosaidmntrolgrid.nectiiier,.cimuitmeam 30lv 2,392,218;-

1 audioampliertube for rectifyng a-- portion of# theoutputi of,r said?. amplier, *and circuit means forsupplyingawagb able biasftosaid audiogamplinerfroxnsaidrectier circuitI means` inversely varying-withi respecty` to; th'eoutput ofsai*d-amplier; wherebyunduewariag tions in the Vvolume of*the-modulatingfrequencg are-compressedf bycont-rollinghe'electiyegamplii-v fication-ofsaidaaudioamplier,

711 Inal volume;I compressing; organization,- 1 an;`

having ai plate 'anglai vari,L ab1e-mu control grid; atransf ormerv couprled gaudio; input-circuit1for -said cont-rolgrigi;v a rectifier.-:cir cuit-means connected to the plate osaidf'amphli'- er for, feeding a portion of ,ts outp].1tj t0; Slld' rectier, and' filteri circuiti` meansv havingJ a time cons tanti ofja: fraction ofgawsecond'forv supplxingg the output ofAv said rectier,v asa negatiye bias on"l the'control'grideoffsaid* amplifier,wherebyF varia; tions inthe volume-=ofthe audio inputgtofsaiclj contro1fgridj'are compressed*` to keep, the output;

of" said`v amplifier 1 moriegnear1y constant LOI-IN L'C:

REFERENCE S'-l CITED* The following references@ are: 0f;L rewrdiz inv; that 'lev of; patenti:

UMTED- STATES-PATENTS- Naxne 

